Mechanistic analysis of activity-dependent BMP/TGF release at a model synapse How do cells diversify signaling outputs of widely used intercellular signaling pathways? A handful of conserved growth factor pathways mediate communication between cells throughout life. Yet remarkably, the signaling outcomes of these pathways are finely tuned to developmental and cellular context. We will investigate signaling specificity of the BMP/TGF pathway. This pathway serves a number of independent functions at the Drosophila NMJ. In particular, it regulates both NMJ morphological growth and neurotransmitter release. We present evidence that the cellular source of the BMP ligand Gbb discriminates between these two pathways. We propose a model in which the single-pass transmembrane protein Crimpy is a sorting receptor for the BMP for dense core vesicles in the regulated secretory pathway. We argue that (1) activity-dependent re- lease of the BMP promotes neurotransmission and (2) Crimpy defines the neuron-derived ligand pool. BMP/TGF family members are synaptically localized and subject to activity-dependent release in mammalian neurons. However, the cellular mechanisms responsible for their localization to dense core vesicles are opaque. Crimpy would represent the first BMP/TGF dense core vesicle sorting receptor identified in any system. Given clinical interest in identifying tools to selectively target synaptic functions of growth factor signaling pathways, a mechanistic understanding of Crimpy and its mammalian homologs will be of significant interest.